Docker frequently asked questions (FAQ)

Does Docker run on Linux, macOS, and Windows?

You can run both Linux and Windows programs and excutables in Docker containers. The Docker platform runs natively on Linux (on x86-64, ARM and many other CPU architectures) and on Windows (x86-64).

Docker Inc. builds products that let you build and run containers on Linux, Windows and macOS.

What does Docker technology add to just plain LXC?

Docker technology is not a replacement for LXC. “LXC” refers to capabilities of
the Linux kernel (specifically namespaces and control groups) which allow
sandboxing processes from one another, and controlling their resource
allocations. On top of this low-level foundation of kernel features, Docker
offers a high-level tool with several powerful functionalities:

Portable deployment across machines. Docker defines a format for bundling
an application and all its dependencies into a single object called a container. This container can be
transferred to any Docker-enabled machine. The container can be executed there with the
guarantee that the execution environment exposed to the application is the
same in development, testing, and production. LXC implements process sandboxing, which is an important pre-requisite
for portable deployment, but is not sufficient for portable deployment.
If you sent me a copy of your application installed in a custom LXC
configuration, it would almost certainly not run on my machine the way it does
on yours. The app you sent me is tied to your machine’s specific configuration:
networking, storage, logging, etc. Docker defines an abstraction for
these machine-specific settings. The exact same Docker container can
run - unchanged - on many different machines, with many different
configurations.

Application-centric. Docker is optimized for the deployment of
applications, as opposed to machines. This is reflected in its API, user
interface, design philosophy and documentation. By contrast, the lxc helper
scripts focus on containers as lightweight machines - basically servers that
boot faster and need less RAM. We think there’s more to containers than just
that.

Versioning. Docker includes git-like capabilities for tracking successive
versions of a container, inspecting the diff between versions, committing new
versions, rolling back etc. The history also includes how a container was
assembled and by whom, so you get full traceability from the production server
all the way back to the upstream developer. Docker also implements incremental
uploads and downloads, similar to git pull, so new versions of a container
can be transferred by only sending diffs.

Component re-use. Any container can be used as a “parent image” to
create more specialized components. This can be done manually or as part of an
automated build. For example you can prepare the ideal Python environment, and
use it as a base for 10 different applications. Your ideal PostgreSQL setup can
be re-used for all your future projects. And so on.

Sharing. Docker has access to a public registry on Docker
Hub where thousands of
people have uploaded useful images: anything from Redis, CouchDB, PostgreSQL to
IRC bouncers to Rails app servers to Hadoop to base images for various Linux
distros. The registry also includes an official “standard
library” of useful containers maintained by the Docker team. The registry itself
is open-source, so anyone can deploy their own registry to store and transfer
private containers, for internal server deployments for example.

What is different between a Docker container and a VM?

Do I lose my data when the container exits?

Not at all! Any data that your application writes to disk gets preserved in its
container until you explicitly delete the container. The file system for the
container persists even after the container halts.

How far do Docker containers scale?

Some of the largest server farms in the world today are based on containers.
Large web deployments like Google and Twitter, and platform providers such as
Heroku run on container technology, at a scale of hundreds of
thousands or even millions of containers.

How do I connect Docker containers?

How do I run more than one process in a Docker container?

This approach is discouraged for most use cases. For maximum efficiency and
isolation, each container should address one specific area of concern. However,
if you need to run multiple services within a single container, see
Run multiple services in a container.

What platforms does Docker run on?

Linux:

Any distribution running version 3.10+ of the Linux kernel

Specific instructions are available for most Linux distributions, including
RHEL, Ubuntu,
SuSE, and many others.

Microsoft Windows:

Windows Server 2016

Windows 10

Cloud:

Amazon EC2

Google Compute Engine

Microsoft Azure

Rackspace

How do I report a security issue with Docker?

You can learn about the project’s security policy
here and report security issues to this
mailbox.

Why do I need to sign my commits to Docker with the DCO?

When building an image, should I prefer system libraries or bundled ones?

Virtually all programs depend on third-party libraries. Most frequently, they
use dynamic linking and some kind of package dependency, so that when
multiple programs need the same library, it is installed only once.

Some programs, however, bundle their third-party libraries, because they
rely on very specific versions of those libraries.

When creating a Docker image, is it better to use the bundled libraries, or
should you build those programs so that they use the default system libraries
instead?

The key point about system libraries is not about saving disk or memory space.
It is about security. All major distributions handle security seriously, by
having dedicated security teams, following up closely with published
vulnerabilities, and disclosing advisories themselves. (Look at the Debian
Security Information for an example of those procedures.) Upstream developers, however, do
not always implement similar practices.

Before setting up a Docker image to compile a program from source, if you want
to use bundled libraries, you should check if the upstream authors provide a
convenient way to announce security vulnerabilities, and if they update their
bundled libraries in a timely manner. If they don’t, you are exposing yourself
(and the users of your image) to security vulnerabilities.

Likewise, before using packages built by others, you should check if the
channels providing those packages implement similar security best practices.
Downloading and installing an “all-in-one” .deb or .rpm sounds great at first,
except if you have no way to figure out that it contains a copy of the OpenSSL
library vulnerable to the Heartbleed bug.

Why is DEBIAN_FRONTEND=noninteractive discouraged in Dockerfiles?

When building Docker images on Debian and Ubuntu you may have seen errors like:

unable to initialize frontend: Dialog

These errors don’t stop the image from being built but inform you that the
installation process tried to open a dialog box, but couldn’t. Generally,
these errors are safe to ignore.

Some people circumvent these errors by changing the DEBIAN_FRONTEND
environment variable inside the Dockerfile using:

ENV DEBIAN_FRONTEND=noninteractive

This prevents the installer from opening dialog boxes during installation which
stops the errors.

While this may sound like a good idea, it may have side effects. The
DEBIAN_FRONTEND environment variable is inherited by all images and
containers built from your image, effectively changing their behavior. People
using those images run into problems when installing software
interactively, because installers do not show any dialog boxes.

Because of this, and because setting DEBIAN_FRONTEND to noninteractive is
mainly a ‘cosmetic’ change, we discourage changing it.

If you really need to change its setting, make sure to change it back to its
default value
afterwards.

Why do I get Connection reset by peer when making a request to a service running in a container?

Typically, this message is returned if the service is already bound to your
localhost. As a result, requests coming to the container from outside are
dropped. To correct this problem, change the service’s configuration on your
localhost so that the service accepts requests from all IPs. If you aren’t sure
how to do this, check the documentation for your OS.

Why do I get Cannot connect to the Docker daemon. Is the docker daemon running on this host? when using docker-machine?

This error points out that the docker client cannot connect to the virtual
machine. This means that either the virtual machine that works underneath
docker-machine is not running or that the client doesn’t correctly point at
it.

To verify that the docker machine is running you can use the docker-machine ls
command and start it with docker-machine start if needed.